Download PDF - Speleogenesis
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20<br />
NCKRI Special Paper No. 1<br />
provide ideal local vertical hydraulic connections between<br />
lateral stratiform passageways for groundwater flow, and<br />
hence significantly affect basinal flow pattern.<br />
Figure 9. A cupola at the ceiling of the uppermost story of<br />
Caverns of Sonora, Texas (view from below; breadth of the photo<br />
is about 2 m). Numerous cupolas at this story open up into a<br />
distinct horizon of touching-vugs type porosity (“burrowed bed”),<br />
which served as a receiving aquifer during the formation of the<br />
cave. Developed at four stories within a vertical range of about 35<br />
m in the layered carbonate Edwards Group, the cave passages are<br />
mainly controlled by fractures encased in distinct beds of compact<br />
limestone, although some more prominent fractures, and hence<br />
passages, cross through several beds.<br />
3.5 Recharge, cave-forming flow and<br />
discharge in hypogene settings<br />
The mode of recharge and discharge, and relationships<br />
between the respective boundaries and zones, are among<br />
the major factors that determine the style of speleogenesis<br />
and resultant cave patterns. In hypogenic speleogenesis,<br />
recharge of water to a given soluble formation occurs from<br />
the adjacent formation below, the main criterion for<br />
distinguishing this class of speleogenesis. Another<br />
important difference is that discharge occurs also through<br />
non-soluble formations. Hydraulic properties of adjacent<br />
formations, and of a major upper confining formation,<br />
impose important controls on speleogenesis in confined<br />
settings (Klimchouk, 2000a; 2003a).<br />
To prevent confusion that arises when referring to<br />
stratigraphic units in terms of their solubility, the<br />
formation that receives recharge from below and hosts<br />
hypogenic caves will be referred to as a cave formation, or<br />
a cave unit, the underlying source formation is a feeding<br />
formation, and the overlying formation into which<br />
discharge occurs is the receiving formation. All of the<br />
formations can be generally soluble, but still with<br />
drastically different capacities to support cave<br />
development under given physical, chemical and<br />
hydrokinetic conditions.<br />
The mode of recharge, in terms of its lateral<br />
distribution, depends on the types, distribution and<br />
connectivity of the original porosity systems in both the<br />
feeding formation and the cave formation as well as the<br />
overall hydrostratigraphy of the system. In the feeding<br />
formation, effective porosity at the contact can be diffuse<br />
and homogenous (A1-A3 in Figure 11), diffuse and<br />
inhomogeneous (zones of enhanced conductivity in<br />
otherwise permeable media; B1-B3), or localized<br />
(tectonically disrupted zones, e.g. fault zones, etc.) In the<br />
latter case, high conductivity zones in the feeding and<br />
receiving formations are commonly co-planar with the<br />
respective major permeability paths across the whole cave<br />
formation (A4). More commonly, there is a disparity of<br />
permeability structures between the feeding and receiving<br />
formations.<br />
Such a disparity is almost always the case at the lower<br />
contact of the cave formation, at its recharge boundary<br />
(Figure 10). Permeability in the feeding formation can be<br />
represented by various combinations of matrix, touchingvug,<br />
fracture or conduit (prominent cross-bedding<br />
fractures) porosity systems, but it never matches the<br />
original permeability structure in the cave formation. The<br />
extreme case of such a disparity is where there are virtually<br />
no hydraulically efficient original flowpaths available in<br />
the cave unit at its lower contact, and hence no perceptible<br />
forced flow through it (Figure 10, F-G). Still, hypogenic<br />
speleogenesis can operate in this situation through the free<br />
convection mechanism.<br />
Vertically across the cave formation, original flow<br />
paths are almost always composed of segments of different<br />
porosity styles and types, as discussed and exemplified in<br />
the previous section. In fact, the relations discussed above<br />
and shown in Figure 10 may be found between adjacent<br />
horizons/units within the inhomogeneous cave formation<br />
itself (see Figure 9). In addition to distinct characteristics<br />
of different porosity segments, the connectivity constraints<br />
between them impose strong effects on speleogenesis.<br />
Complex 3-D structural organization of the most widely<br />
acknowledged ascending hypogenic caves, such as gypsum<br />
mazes in the western Ukraine, limestone mazes of the<br />
Black Hills, South Dakota, USA, composite pattern caves<br />
of the Buda Hills in Hungary and the Capitan reef complex<br />
of the Guadalupe Mountains, New Mexico, USA, illustrate<br />
the effect of this heterogeneity. It is important to<br />
underscore that because hypogenic speleogenesis is often<br />
the product of mixed (topography-driven and densitydriven)<br />
flow systems, buoyancy effects are commonly<br />
involved in establishing hydraulic connections between<br />
different porosity segments (Section 3.8).